Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, digital cameras, digital recording devices, video gaming devices, video game consoles, cellular or satellite radio telephones, and the like. Digital video devices implement video compression techniques, such as MPEG-2, MPEG-4, or ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), to transmit and receive digital video information more efficiently. Video compression techniques perform spatial prediction and temporal prediction to reduce or remove redundancy inherent in video sequences.
Intra-coding relies on spatial prediction to reduce or remove spatial redundancy between video blocks within a given coded unit, which may comprise a video frame, a slice of a video frame, a group of pictures, or another defined unit of video blocks. In contrast, inter-coding relies on temporal prediction to reduce or remove temporal redundancy between video blocks of successive coded units of a video sequence. For intra-coding, a video encoder performs spatial prediction to compress data based on other data within the same coded unit. For inter-coding, the video encoder performs motion estimation and motion compensation to track the movement of corresponding video blocks of two or more adjacent coded units.
A coded video block may be represented by prediction information that can be used to create or identify a predictive block, and a residual block of data indicative of differences between the block being coded and the predictive block. In the case of inter-coding, one or more motion vectors are used to identify the predictive block of data, while in the case of intra-coding, the prediction mode can be used to generate the predictive block. Both intra-coding and inter-coding may define several different prediction modes, which may define different block sizes and/or prediction techniques used in the coding. Additional types of syntax elements may also be included as part of encoded video data in order to control or define the coding techniques or parameters used in the coding process.
After block-based prediction coding, the video encoder may apply transform, quantization and entropy coding processes to further reduce the bit rate associated with communication of a residual block. Transform techniques may comprise discrete cosine transforms or conceptually similar processes. Alternatively, wavelet transforms, integer transforms, or other types of transforms may be used. In a discrete cosine transform (DCT) process, as an example, the transform process converts a set of pixel values into transform coefficients, which may represent the energy of the pixel values in the frequency domain. Quantization is applied to the transform coefficients, and generally involves a process that limits the number of bits associated with any given transform coefficient. Entropy coding comprises one or more processes that collectively compress a sequence of quantized transform coefficients.
Context-adaptive variable length coding (CAVLC) is one type of entropy coding technique that is currently supported in some coding standards, such as the International Telecommunication Union Standardization Sector (ITU-T) H.264/Motion Pictures Expert Group (MPEG)-4, Part 10 Advanced Video Coding (AVC) (hereinafter “H.264/MPEG-4 Part 10 AVC standard”). In accordance with CAVLC as defined in the H.264/MPEG-4, Part 10 AVC standard, an encoder encodes a plurality of syntax elements that represent a coefficient vector. The syntax elements may include a total number of coefficients in the coefficient vector, a number of trailing ones in the coefficient vector, signs of each of the trailing ones, magnitudes (or levels) of nonzero coefficients other than the trailing ones, sum of all runs (i.e., the total number of runs before the last nonzero coefficient), and a run length of zeros before each nonzero coefficient. CAVLC is adaptive in that the encoder adaptively selects the VLC coding tables to use in encoding the syntax elements based on particular characteristics of the coefficient vector.